Epidemiological findings with diseases caused by prions have led to a number of important unresolved issues concerning the induction of sporadic cases. The suggestion has been made that a reservoir and/or vector of infectious agent accounts for the continuing occurrence of sporadic cases. Preliminary results suggest a possible role for hay mites in the occurrence of scrapie on Icelandic farms. Mite preparations from 3 of the t Icelandic farms tested have caused clinical scrapie in a proportion of mice infected by either the intraperitoneal (i.p.) or intracerebral (i.c.) route. Proteinase K (PK) treated brain homogenates from these cases were confirmed as scrapie by Western blot (WB)analysis for PrP5c. In addition, a concentrated mite preparation from one of the positive farms had robust staining for PrP5c, the protein that signals the presence of scrapie. This preparation also yielded the highest proportion of mice positive for transmission. Confirmation of the above findings will be obtained utilizing mite preparations from farms on which scrapie is endemic and from additional sheep farms which are free of scrapie disease. Homogenates from these preparations will be assayed for scrapie infectivity by injection of mice and for PrP5c by WB, ELISA and immunohistochemistry. In additional studies we will analyze mite preparations for PrP and for the genetic information that encodes PrP by PCR. We will also determine the molecular weight of normal mite PrP by PAGE and WB and will assess the PK sensitivity of the normal protein. The importance of this finding in the epidemiology of these diseases, including sporadic CJD, warrants a closer examination of the relationship between the causative agents of prion infections and arachnid vectors such as mites. The existence of a mite reservoir/vector would have far reaching implications on the epidemiology, transmission and control of prion disorders.